Exhaust gas purifying catalysts generally have noble metal particles carried on the surfaces of metal oxide particles to convert toxic components (e.g., unburned hydrocarbon (HC) or carbon monoxide (CO)) contained in an exhaust gas into nontoxic components such as water, carbon dioxide (CO2) and the like through oxidation with the noble metal particles.
In recent years, there has been an increasing demand for an exhaust gas purifying catalyst capable of performing highly efficient purification of unburned HC or carbon monoxide to satisfy regulations concerning automobile emissions, which have become increasingly strict. In response to such a demand, various efforts have been made to improve the exhaust purifying catalyst. For example, one of the conventional methods of producing the exhaust gas purifying catalyst is based on the fact that an increase in surface area of noble metal particles contained in the catalyst leads to an improvement in the purifying capabilities of the catalyst. In this method, the surface area of the noble metal particle is increased in order to increase surface energy by reducing the diameter of the noble metal particle contained in the catalyst, thereby improving the performance of the catalyst.
In this process, the noble metal particle for the exhaust gas purifying catalyst is in an ultra fine state of several nanometers or less at an initial stage. However, when the catalyst is put into a practical use and exposed to an oxidative atmosphere at high temperatures, the surface of the noble metal particle is oxidized. Further, adjacent noble metal particles are coagulated and fused together to form a coarsened particle of several dozen nanometers to thereby decrease the surface area of the noble metal particles, thus causing a periodic reduction in the rate at which the catalyst purifies the toxic substances.
Japanese Patent Laid-open Publication No. 2000-15097 discloses an exhaust gas purifying catalyst having noble metal particles uniformly dispersed in whole catalyst layers formed inside a substrate, through which plural fine pores are formed in a honeycomb shape, by preventing coagulation of the noble metal particles. One method produces catalyst powder by mixing a noble metal colloid solution with a metal alkoxide, followed by decomposition of the metal alkoxide via hydrolysis.
In addition, as a method of preparing a noble metal particle having a high surface area to ensure higher activity by preventing the noble metal particle from being coarsened and decreased in surface area, a reversed micelle method has been developed in the art. In the reversed micelle method, after preparing an emulsion in which a reversed micelle of a solution containing raw noble metal particles is formed, a particulate noble metal is precipitated from the reversed micelle. The reversed micelle is then disrupted to obtain the precipitates, which in turn are treated by a series of processes such as filtering, drying, grinding and firing to provide a catalyst.
According to a reversed micelle method disclosed in Japanese Patent Laid-open Publication No. 2005-111336, a heat-resistant catalyst is prepared by mixing a reversed micelle solution containing a noble metal colloid solution in a micelle, a reversed micelle solution containing a metal hydroxide solution in a micelle and a metal alkoxide, followed by firing the mixture. Another reversed micelle method disclosed in Japanese Patent Laid-open Publication No. 2005-185969 includes a process of adjusting a reversed micelle solution containing a solution of noble metal salts and a solution of metal salts as at least one co-catalyst component on the same base material in order to provide a highly heat-resistant catalyst.